Fixed https://github.com/couchbase/couchbase-lite-android/issues/888
Fixed https://github.com/couchbase/couchbase-lite-android/issues/888

Issue: Crash during 1.1 to 1.2 migration causes permanent loss of data

Changes:
- Instead of migrate to actual new name, create temporary database and migrate to temporary db, then rename temporary name to actual name.
- If temporary database exists, rollback it to old name before continue to migration. Rollback actually rollback attachment files.
- Because of calling backOut() for rollback, importData() method do move file attachment instead of copy file. <- No change.

couchbase/couchbase-lite-java-core